1. Field of the Invention
The present invention relates to photolithography. More particularly, the present invention relates to a method of aligning a wafer in photolithographic exposure equipment.
2. Description of the Related Art
Photolithography is a technique used for transcribing patterns onto a wafer. In photolithography, an alignment process is first performed to align a wafer with exposure equipment. More specifically, the exposure equipment includes a reticle bearing a pattern that is to be transferred to the wafer. The alignment process is for aligning a pattern that has already been formed on the wafer with the pattern of the reticle. The alignment process is carried out using an alignment mark formed on the wafer. Subsequently, an exposure process is carried out in which the wafer is exposed to light directed through the reticle such that an image corresponding to the pattern of the reticle is transferred to the wafer.
However, despite the alignment process, an overlay error or a misalignment error may occur between the image transferred to the wafer and a pattern already formed on the wafer. To prevent such errors from re-occurring, an overlay measurement process is performed after the exposure process to generate overlay data representative of the degree of mis-alignment between the wafer and the reticle. Then, an error correction value is produced from the overlay data, and the error correction value is fed back to the exposure equipment for use in controlling the next alignment process. That is, the alignment data used to control the alignment process is updated with feedback from the overlay measurement process.
FIG. 1 is a schematic diagram of the feedback relationship between an alignment apparatus and an overlay measurement apparatus in conventional exposure equipment.
Referring to FIG. 1, an exposure apparatus 10 performs an alignment and alignment modeling process 11 for aligning a wafer loaded thereon and modeling the alignment, and then performs an actual exposure process 13 on the aligned wafer. After the exposed wafer is unloaded from the exposure apparatus 10, the wafer is loaded on an overlay measurement apparatus 20, and the overlay measurement apparatus 20 performs an overlay measurement and overlay modeling process 21 for the wafer. The overlay measurement results (correction value) are fed back to the exposure apparatus 10 for use in correcting the wafer alignment process. The overlay modeling process calculates the correction value on the basis of the measured overlay data. The alignment modeling process is for applying the feedback correction value to the alignment data used to control the alignment process.
Such overlay and alignment modeling processes accompanying the overlay measurement and alignment processes are performed after each time a wafer is exposed. Accordingly, a great deal of time is required to manufacture semiconductor devices or the like using the conventional photolithography process.